Some human activities cause underwater noise that is transmitted from the source of the underwater noise to the surrounding environment, sometimes many miles away. The underwater noise generated by oil and gas drilling platforms, ships and other human activities and machinery is generally considered undesirable. Some studies conclude that underwater noise pollution can adversely affect marine life, and it may be disruptive to other human activities such as scientific, meteorological and military activities. This is especially true for noise generating activities that result in large amplitude acoustic emissions (loud sounds) and transmissions at frequencies to which human and oceanic life is sensitive.
Ships that operate in environmentally sensitive or highly regulated regions can be limited in the manner or time in which they can operate due to the noise generated by the ship. This occurs in the oil and gas field, where noise from mobile drilling ships limits drilling time due to the effect that the noise can have on migrating bowhead whales in Arctic regions. When bowhead whales are sighted, operations may be halted until they have safely passed, and this process can take many hours.
As mentioned above, there is some concern over the effect that shipping and other man-made noise has on marine mammals. Some studies suggest that man-made noise can have a significant impact on the whale's stress hormone levels, which might affect their reproduction rates, etc.
Known attempts to reduce noise emissions from surface ships include the use of a so-called Prairie Masker, which uses bands of hoses that produce small freely-rising bubbles to mitigate ship's noise. However, small freely-rising bubbles are usually too small to effectively attenuate low-frequency noise. In addition, Prairie Masker systems require continuous pumping of air through the system, a process itself that produces unwanted noise, and also consuming energy and requiring a complex gas circulation system that is costly and cumbersome to the other operations of the ship. Finally, such systems cannot operate below a given depth due to hydraulic forces and back pressures.
One principle that is useful in approximating or understanding the acoustic effects of gas pockets in liquid (e.g., air pockets or bubbles or enclosures in water) is the behavior of spherical gas bubbles in liquid. The physics of gas bubbles is relatively well known and has been studied theoretically, experimentally and numerically.
FIG. 1 illustrates a model of a gas (e.g., air) bubble 10 in a liquid 15 (e.g., water). One model for studying the response of gas bubbles is to model the bubble of radius “a” as a mass on a spring system. The mass is “m” and the spring is modeled as having a spring constant “k”. The radius of the bubble 10 will vary with pressures felt at its walls, causing the bubble 10 to change size as the gas therein is compressed and expands. In some scenarios the bubble 10 can oscillate or resonate at some resonance frequency, analogous to how the mass on spring system can resonate at a natural frequency determined by said mass, spring constant and bubble size.
Continuing efforts to mitigate the effects of underwater noise continue. While some solutions can actually reduce the amount of noise generated by a source other solutions seek to reduce the effect of the noise by surrounding or partially surrounding the noise-making source with something that absorbs or otherwise attenuates the propagated noise.